Evolved Packet System (EPS) is the evolved 3rd Generation Partnership Project (3GPP) Packet Switched Domain. EPS includes Evolved Packet Core (EPC), and Evolved Universal Terrestrial Radio Access Network (E-UTRAN) or Long Term Evolution (LTE), and is sometimes also referred to as a fourth generation (4G) system. An EPC architecture comprises a Packet Data Network (PDN) Gateway (PGW), a Serving Gateway (SGW), a Policy and Charging Rules Function (PCRF), and a Mobility Management Entity (MME). The radio access, E-UTRAN, consists of one or more eNodeBs (eNB), serving wireless devices also called User Equipment (UE).
FIG. 1 shows the overall E-UTRAN architecture and includes eNBs, providing the E-UTRA user plane and control plane protocol terminations towards the UE. The user plane control terminations comprise Packet Data Convergence Protocol (PDCP), Radio Link Control (RLC), Medium Access Control (MAC), and a Physical Layer (PHY). The control plane control terminations comprise Radio Resource Control (RRC) in addition to the listed user plane control terminations. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of the S1 interface to the EPC, more specifically to the MME by means of the S1-MME interface and to the SGW by means of the S1-U interface. The main parts of the EPC Control Plane and User Plane architectures are shown in FIG. 2 and FIG. 3 respectively.
Integration of new radio technologies to legacy ones has always been an important feature in any wireless communication generation shift. A future fifth generation (5G) Radio Access Technology (RAT) is preferably tightly integrated with another RAT, such as the EPS/4G RAT. The envisioned tighter integration may lead to architecture problems that have to be solved. In this context, the tight integration means that common protocol layer(s) running on the top of RAT-specific protocol layers of LTE and of the new 5G air interface will be specified.
System Control Plane Concept
For the purpose of 5G, a clean slate radio network solution that has been designed as described in the article “A Clean Slate Radio Network Designed for Maximum Energy Performance” published at IEEE PIMRC 2014, with the aim to maximize energy performance, simplify usage of advanced antenna systems, and reduce interference. A logical separation between idle mode functions and user plane data transmission and reception is proposed. This separation allows meeting the key challenge of reducing idle mode energy consumption in the network by designing for Discontinuous transmission and reception (DTX and DRX) also in the network nodes. The clean slate energy optimized system design targets this challenge with a new way of transmitting system information and performing initial system access. A 5G System Control Plane (SCP) is designed to be ultra-lean and static. When there are no data transmissions there should be as little mandatory network transmissions as possible. Furthermore all nodes do not need to participate in the distribution of system information.
An SCP should have a very limited number of responsibilities. Among the few functions that are supported by the SCP are Random Access (RA), including distribution of access information, and paging, including passive mode mobility and location area update. The information related to initial system access is broadcasted in an access information table (AIT). A particular AIT may contain information relating to several different nodes in the network. A system signature sequence associated with a system signature index (SSI) is transmitted from a network node and the received SSI can then be used by a wireless device to select the relevant access information from the AIT. Everything else is defined on a per need basis, such as the format used for transmission of user plane data, and can therefore be optimized for the active users without any concern on how that affect idle mode users. RA and paging are perhaps the most basic functions in a mobile broadband network. In order to perform a RA the UEs need to receive some information on how to do that. That information is denoted access information. In order to support paging functions like location area update and idle mode mobility, additional UE paging configuration (UEPC) is needed. The UEPC is sent in a dedicated fashion only to UEs that should be possible to page from the network. The UEPC typically contain information on:
The configuration of the downlink paging channel;
How to determine the current tracking area code (TAC) identity;
When and how to perform a location area update.
Problems with Existing Solutions
Today the UE needs to ensure that it camps on the “best” cell and RAT in order to be best served when it uses the network services. The “best” cell or RAT is determined using a number of specified criteria. This is important to avoid a cell reselection or handover immediately after the UE has set up a connection and has requested the particular service. For example, a UE which prefers the 5G RAT since it wants to use 5G services, needs to camp on cells for the 5G RAT whenever these cells are available. However, newer RATs typically have spotty coverage, and this will probably be the case for the 5G RAT when it is newly deployed. Therefore, enforcing the UE to camp on the 5G RAT when possible may imply that frequent measurements and cell reselection evaluations are needed, thus leading to UE power drain. Furthermore, it may imply that a large number of location area update signaling messages are needed as the UE move in and out of the 5G RAT coverage, thus leading to high signaling load on the network.
In order to reduce UE complexity, save UE power and reduce signaling, it is therefore beneficial if the UE camps in a single RAT such as in LTE which has a better coverage than the 5G RAT. This is the case already in existing multi-RAT scenarios, and will be useful also in the case of the 4G-5G tight integration. If the UE camps on a given RAT and receives a paging message in this RAT, the UE will currently respond in this RAT and in the same cell where it received the page.
However, this means that the UE will not always initiate a connection in the optimal RAT, which in turn increases overhead and to some extent user session establishment delay if a handover immediately after connection establishment is needed. Furthermore, in the SCP design the UE does not camp on any cell, as the cell concept is not the same as in a conventional network design. If there are no cells then connecting a UE to the node that transmitted a paging message is not trivial. The UE may e.g. instead access a small cell that is controlled by a node which does not have any data buffered to the UE because the data may instead be located in a node controlling a macro cell.